IgE antibodies appear important in protective immunity to helminths. Their evaluation in patients with filariasis, onchocerciasis, schistosomiasis and strongyloidiasis indicates that variations in individual responses reflect differences in regulation of antibody production rather than differences in recognition of immunogens. Clinical studies have defined a state of diminished immune responsiveness to parasite antigens in these helminth infections and in leishmaniasis and trypanosomiasis (Chagas). The mechanisms responsible for cellular immune suppression include serum factors, adherent mononuclear cells and histamine while those responsible for limiting immediate hypersensitivity responses involve specific IgG blocking antibodies. After chemotherapy these suppressor mechanisms markedly diminish or disappear. Immunodiagnostic studies have identified specific antigens and antibody systems able to distinguish the type, stage and degree of infection for several infections. Strain differences defined among the protozool parasites, leishmania and giardia, are important in determining both the character of the host response evoked in infected individuals and their response to treatment.